Spherodization of grain boundary precipitates

ABSTRACT

A METHOD FOR SPHEROIDIZING GRAIN BOUNDRY PRECIPITATES BY CYCLING THROUGH THE ALPHA-GAMMA PHASE TRANSFORMATION, WHEREIN BOTH THE DUCTILITY AND STRENGTH OF THE MATERIAL ARE INCREASED. THE INVENTIVE SPHEROIDIZATION PROCESS IS APPLICABLE TO THE BINARY SYSTEM, SUCH AS FE-TA. IN ADDITION IT HAS BEEN DISCOVERED THAT THE ADDITION OF SELECTED AMOUNTS OF CHROMIUM ON THE FE-TA BINARY SYSTEM (FETA-CR ALLOY) ENHANCED SPHEROIDAZATION AS WELL AS IMPROVED OXIDATION RESISTANCE, WHILE LOWERING THE HEAT TREATMENT TEMPERATURE.

Feb. 26, 1974 R. H. JONES ETAL 3,794,532

SPHEROIDIZATION OF GRAIN BOUNDARY PRECIPITATES Filed March 1-4, 1972 2 Sheets-Shout 1 SOLUTION TREATMENT IN 8 PHASE FIELD I ZATION Y I Fez TO AN NEALING TREATMENT GRAIN SIZE CONTROL "DURING y-a TRANSFORMATION GRAIN REFINEMENT AND PARTICLE SPHEROIDI TREATMENT IN PHASE FIELD l O O 0 AGING TREATMENT IN a Fe T0 PHASE FIELD I 6OMIN TIME (MINUTES) O 2 YIELD STRESS, ksi

TEST TEMPERATURE, c

Feb. 26, 1974 R. H. JONES E'I'AL 3,794,582

SPHEROIDIZATION OF GRAIN BOUNDARY PRECIPITATBS Filed larch 14, 1972 2 She9ta-$heot I 8 Fe To 1300 TEMPERATURE 7+ Fe To a+Fe To TANTALU M, M.%

United States Patent O 3,794,532 SPHERODIZATION F GRAIN BOUNDARY PRECIPITATES Russell H. Jones, Pittsburgh, Pa., and Earl R. Parker,

Oakland, and Victor F. Zackay, Berkeley, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Mar. 14, 1972, Ser. No, 234,483 Int. Cl. C22c 39/14, 41/02 US. Cl. 148--142 8 Claims ABSTRACT OF THE DISCLOSURE A method for spheroidizing grain boundary precipitates by cycling through the alpha-gamma phase transformation, wherein both the ductility and strength of the material are increased. The inventive spheroidization process is applicable to the binary system, such as Fe-T a. In addition, it has been discovered that the addition of selected amounts of chromium on the Fe-Ta binary system (Fe- Ta-Cr alloy) enhanced spheroidization as well as improved oxidation resistance, while lowering the heat treatment temperature.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, USAEC contract W7405'Eng48 with the United States Atomic Energy Commission.

This invention relates to the heat treatment of steels, particularly to the spheroidization of the grain boundary precipitates in Fe-Ta binary systems, and more particularly to the spheroidization of the grain boundary precipitates in 'FeTa-Cr alloys.

There has been much prior effort to derive relationships which describe the yielding behavior and work hardening behavior of single phase alloys. The yield strength has been related to dislocation morphology, alloy content, grain size, internal stresses, particle morphology and many other variables. Also, the work hardening of single phase alloys has been related to grain size, dislocation density and stacking fault energy.

In two phase alloys containing hard particles the largest amount of prior effort has been in the analysis of dislocation particle interaction mechanisms for yielding, with less emphasis placed on the analysis of work hardening mechanisms in alloys containing hard particles.

Two phase alloys containing a dispersion of hard particles have been produced in the three following ways:

(1) nucleation and growth, (2) internal oxidation, and (3) powder metallurgical techniques. As known, a complex grain boundary structure may result from the nucleation and growth process in polycrystalline materials. This boundary region may have a heavy grain boundary network of the second phase with a precipitate free zone adjacent to the network. A grain boundary structure such as this would alter the yield and flow behavior of an alloy when compared to an alloy without this grain boundary structure.

SUMMARY OF THE INVENTION The present invention overcomes the prior problems of the above-mentioned grain boundary structure by providing a method which involves an allotropic phase change in binary systems after the aging treatment, which refined the grain structure and spheroidized the grain boundary network. The final structure produced by the invention is a random dispersion of particles in a soft polycrystalline matrix with the grain boundary network spheroidized and no longer positioned at a grain boundary. It has been found that this spheroidization can be carried out in binary systems, such as Fe-Ta, and ternary systems, such as 3,794,532 Patented Feb. 26, 1974 Fe-Ta-Cr. Both the ductility and strength are increased by the spheroidization treatment.

Therefore, it is an object of this invention to provide a method for spheroidization of the grain boundary precipitates in alloys.

A further object of the invention is to provide a process which grain boundary precipitates are fully spheroidized by cycling through the u-v phase transformation.

Another object of the invention is to provide a method which produces a material having enhanced oxidation corrosion resistance, utilizes lower heat treating temperature, and minimize embrittlement of grain boundaries.

Another object of the invention is the producing of an alloy, either binary or ternary, which has spheroidized grain boundary network having a random dispersion of particles.

Another object of the invention is to provide an Fe- Ta-Cr alloy wherein the grain boundary precipitates are fully spheroidized.

Other objects of the invention will become readily apparent from the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart illustrating the heat treating cycle which includes the novel spheroidization technique.

FIG. 2 is an equilibrium phase diagram for the Fe-Ta binary system.

FIG. 3 illustrates the grain boundary network without the spheroidization process.

FIG. 4 illustrates the effect of the spheroidization process on the grain boundary of FIG. 3.

FIG. 5 is a graph showing temperature vs. yield stress of various materials including the novel Fe-Ta-Cr alloy.

DESCRIPTION OF THE INVENTION Since spheroidization of the grain boundary precipitates in alloys is applicable in binary systems, such as Fe-Ta, and ternary systems, such as Fe-Ta-Cr. the novel spheroidization technique will first be described with respect to the binary system to illustrate the substantial advancement made in the art by this invention.

In the Fe-Ta binary embodiment utilized in carrying out the novel spheroidization process, the alloys were cast from 99.95% purity electrolytic iron and 99.9% initial purity tantalum. The composition preparation of the alloy does not constitute part of this invention and is not described in detail, but is set forth in coinventor Jones Ph.D. Thesis dated October 10, 1971, LBL-l79, entitled Prediction of the Stress-Strain Behavior of Polycrystalline Ot-II'OII Containing Hard Spherical Particles. The heat treating cycle utilized is schematically shown in FIG. 1.

Knowledge of the equilibrium phases which are present in an alloy may be found from the equilibrium phase diagram shown in FIG. 2.

An understanding of the structural characteristics of the two phase alloys utilized in the tests conducted requires an examination of the alloys during each stage of processing. The homogeneity of any two phase structure is dependent upon the segregation which occurs during solidification. The slow diffusion rate of tantalum in iron would require a very high temperature for homogenizing, therefore small diameter ingots were used to minimize segregation during solidification. After casting, the ingots were hot worked to a 0.50 in. by 0.50 in. shape at 1000 C. This hot working temperature was chosen to reduce texturing from the deformation process.

The second phase was formed, after solution treating at 1400 (3., shown as the first leg of FIG. 1, and quenching to room temperature, by the reaction: a(supersat.) x+F6 Ta at 700 C. The Fe Ta particles which formed during this reaction where plate shaped and there was a heavy network of the Laves phase at the grain boundaries, as shown in FIG. 3. The alloy was then age treated as indicated by the second leg of FIG. 1. The treatment provided by the inventive technique was accomplished by heating the alloy to 1100 C., as shown in FIG. 1 at the third leg of the flow chart, at which the stable structure is 'y+Fe Ta. The eifect of this novel treatment was to spheroidize the matrix and grain boundary particles, and refine the grain size as shown in FIG. 4. The efiect of this inventive treatment on the grain boundary and matrix particles can be seen by comparing FIG. 3 and FIG. 4. Also, after the allotropic phase changes of OL 'ytX the prior grain boundary network is positioned within the grains rather than at the boundaries. The cycle illustrated in FIG. 1 is completed by an annealing treatment as shown by the fourth leg of the chart. Tests, described in detail in the above-referenced Thesis show that both the ductility and strength of the binary alloy were increased by the inventive spheroidization treatment.

While the spheroidization treatment of the binary Fe-Ta system described above has eliminated the brittle fracture characteristic of that alloy, the relatively high solution treating temperature required causes oxidation of the alloy, and coarsening of precipitate particles during the spheroidization treatment occurred.

It has been found that the addition of chromium to the Fe-Ta alloy serves to enhance spheroidization as well as improve oxidation resistance and lowering the heat treatment temperature. Thus an improved high temperature alloy has been developed, namely, an Fe-Ta-Cr alloy. During tests conducted, the composition Fe-Ta-Cr alloy was varied from 1-2% Ta, 3-7% Cr, balance Fe. Dilatometer analyses were done to determine the m y and (2-6 phase transformation temperatures of each alloy, and all specimens were heat treated by encapsulating in quartz tubes (0.3 atmos. pressure of argon gas at room temperature) and solution treated at 1320 C. for one hour, whereafter the specimens were quenched in iced salt water. Specimens for u-v phase transformation temperature determination by the metallographic method were held at 860 C. for 12 hours, and then heated up to 890 C. and 950 C., respectively, at a heating rate of 1 C./minute, followed by quenching into salt water.

Tensile specimens were quenched from 1320 C. (similar to the first leg of FIG. 1) after one hour solution treatment into 45 C. water.

Aging of solution treated specimens were performed in a molten salt bath at 700 C. for 45 minutes (see second leg of FIG. 1). After aging, these specimens were air cooled.

spheroidization treatment (third leg of FIG. 1) was carried out with the specimens in protective steel bags, resulting in a grain structure similar to that of FIG. 4.

Hardness and tensile tests were then conducted and X- ray diffraction pattern analysis of the spherical particles were made.

The dilatometer analysis showed that the 'y-6 phase transformation temperature had been lowered about 60 C. and the a-q phase transformation temperature had been lowered about 100 C. by the addition of chromium to Fe-Ta binary alloys. The results of the metallographic examination agreed reasonably with the dilatometer analysis. Thus chromium up to 7 atomic percent has the efiect of lowering both the rut-*y and -6 transformation temperatures. Further increase in chromium content will lower the -6 transformation temperature but will raise the oz'y transformation temperature.

Experimental results show that for hypoeutectoid alloys (Fe-1Ta-3Cr, and Fe-lTa-SCr), the temperature range of the solid solutions, 6 and 8+ is lowered with increasing chromium content. For hypereutectoid alloys (Fe-lTa-7Cr, Fe-2Ta-3Cr, Fe-2Ta-5Cr, and Fe-2Ta-7Cr), the fl-Laves phase solubility line is shifted to the lower concentrations of Ta. The eutectoid composition was also shifted to lower tantalum concentrations.

The peak hardness of the specimens tested on aging at 700 C. occurred at 40 minutes, which was quite similar to that of binary alloys. This indicates that chromium does not affect the aging characteristics of Fe-Ta binary systems. In Fe-Ta binary systems, the precipitates were rather plate shaped and not spherical, while in the ternary alloys the particles were plate and spherical shaped. The chromium appears to have changed the surface energy, thus causing some spheroidization of grain boundary and matrix particles during the aging treatment.

As pointed out above with respect to the Fe-Ta system,

spheroidization in the binary alloy was accomplished by cycling through the oz'y allotrophic phase transformation. In the ternary aloys, some grain boundary films were already broken up and some particles were rather spherical before the spheroidization treatment. However, the degree of spheroidization was not suflicient to eliminate the brittleness of the lower chromium alloys; thus, it was necessary to use the oc'y transformation treatment for these materials. For the binary alloys, it took 10 minutes to break up the continuous boundaries at 1100 C., but in the ternary alloys, an equivalent spheroidization occurred in less than 10' minutes. In the later, much shorter time could be used to obtain enough spheroidization at 1100 C. and 950 C.

The spheroidization temperature in the 'y+Fe Ta region affects the hardness of the Fe-lTa-SCr alloy, wherein the peak aged hardness was 169, and the spheroidization treatments above 975 C. produced an added increment of hardness.

A significant difierence created by the chromium can be seen on the mechanical properties of aged specimens between the binary and ternary alloys. For the aged condition, the binary alloy showed almost zero elongation while the ternary alloys had elongation values up to 25% (for 7 a/o chromium alloy). The ductility increased with chromium content, the partly spheroidized grain boundary precipitates after aging being responsible for the higher ductility.

For the spheroidized specimens, the strength increased with increasing chromium content and the ductility decreased with increasing chromium content.

spheroidization treatments without protective steel bags at 1100' C. for 10 minutes showed that for 3 Cr and 5 Cr alloys, oxides and scales were observed on the surface of the specimens but for 7 Cr alloys, no scales were observed. Multicycling of the spheroidization treatment produced an increase in ductility and a finer grain size.

FIG. 5 illustrates the yield stress of various alloys including the above described Fe-Ta-Cr alloy. As seen the ternary alloy produces, with the exception of Fe-l at percent Ta Cold Worked Steel, higher yield stress at temperatures below 600 C. and higher above 600 C. except for Hastelloy X, thus further illustrating the advantages of the novel Fe-Ta-Cr alloy.

It has thus been shown that the Fe-Ta-Cr alloy produces the following over the binary Fe-Ta systems:

(1) The 11- phase transformation temperature was lowered about C. by adding not greater than 7 at percent chromium, and the -6 transformation temperature was lowered about 60 0., relative to the binary phase diagram.

(2) The eutectoid composition was shifted by the addition of Cr to the lower concentration of Ta.

(3) The second phase in the ternary system was established as the Laves phase, Fe Ta.

(4) The age hardening characteristics were similar to those of the binary alloys.

(5) The grain boundaries of the aged specimens of the ternary alloys were not completely continuous. Spheroidization in ternary alloys occurred in less than 10 minut (6) The strength of the ternary alloy increased with increasing spheroidizing temperature. The spheroidizati-on treatment can be used as the hardening treatment. In ternary alloys, almost the same strength could be obtained by 950 C. spheroidization; this was 150 lower than the spheroidization temperature for binary alloys.

(7) The strength of the ternary alloys after spheroidization increased with chromium content over 7% and the ductility after spheroidization decreased with chrmium content over 7%. For the aged specimens, the ductility increased with increasing chromium content.

(8) By multicycling, the ductility can be increased and a fine grain size can be obtained.

(9) The oxidation resistance of the ternary alloys increased with increasing chromium content.

It has thus been shown that the present invention provides a spheroidization method and ternary composition which greatly advances the state of art.

While particular embodiments and examples have been illustrated and described modifications and changes will become apparent to those skilled in the art, and it is intended to cover in the appended all such modifications and changes that come within the spirit and scope of the invention.

What we claim is:

1. In a process for treating binary Fe-Ta and ternary Fe-Ta-Cr alloys which includes the steps of: solution treating in the 6 phase field, quenching, age treating in the alpha+Fe Ta phase field, and cooling, the improvement comprising the steps of cycling the thus treated alloy through the alpha-gamma phase transformation causing spheroidization of the grain boundary precipitates.

2. The process defined in claim 1, additionally including the steps of annealing the thus spheroidized alloy by heating to a temperature of about 800 C. for a time period of about 25 minutes, and cooling the thus heated alloy.

3. The process defined in claim 1, wherein the steps of cycling the alloy through the alpha-gamma phase transformation includes the steps of heating the age treated alloy to the gamma+Fe Ta phase field, and cooling the thus heated alloy.

4. The process defined in claim 3, wherein the heating step is carried out in the temperature range between about 950 C. and about 1100 C., and the cooling step is accomplished by partial furnace cooling with the remainder air cooled.

5. The process defined in claim 4, wherein the temperature is maintained for about ten minutes. 6. The process defined in claim 1, additionally including the steps of multicycling the age treated alloy through the alpha-gamma phase transformation to increase ductility and produce finer grain sizes.

7. The process defined in claim 1, additionally includ ing the step of selecting the composition of the alloy to be treated from the group consisting of Fe-1%Ta-3%Cr, Fe- 1 Ta-5 Cr, Fe-1% Ta-7 Cr, Fe-2 Ta-3 Cr, and Fe-2%Ta-5%Cr, Fe-2%Ta-7%Cr.

8. The process defined in claim 1, wherein the cycling steps are carried out by heating the alloy to a temperature of about 950 C. to 1100 C., maintaining the alloy at such temperature for a time period of no greater than 10 minutes, furnace cooling the alloy to a temperature of about 750 C., and air cooling the alloy to ambient temperature.

References Cited UNITED STATES PATENTS 1,711,484 5/1929 Armstrong 126 F 1,389,680 9/1921 McKenna 75--123 I 1,687,486 10/1928 Armstrong 75126 F OTHER REFERENCES Journal of the Iron & Steel Institute, 1936, pp. 173 (p)- 207(1 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

